Zinc/air depolarized cells are typically in the form of miniature button cells which have particular utility as batteries for electronic hearing aids including programmable type hearing aids. Such miniature cells typically have a disk-like cylindrical shape of diameter between about 4 and 12 mm and a height between about 2 and 6 mm. Zinc air cells can also be produced in somewhat larger sizes having a cylindrical casing of size comparable to conventional AAAA, AAA, AA, C and D size Zn/MnO2 alkaline cells and even larger sizes.
The miniature zinc/air button cell typically comprises an anode casing (anode can), and a cathode casing (cathode can). The anode casing and cathode casing each have a closed end an open end and integral side walls extending from the closed end to the open end. The anode casing is fitted with an insulating seal ring which tightly surrounds the anode casing side wall. After the necessary materials are inserted into the anode and cathode casings, the open end of the cathode casing is typically pushed over the open end of the anode casing during assembly so that a portion of the cathode casing side walls covers a portion of the anode casing side wall with insulating seal therebetween. The anode and cathode casing are then interlocked in a second step by crimping the edge of the cathode casing over the insulator seal and anode casing.
The anode casing can be filled with a mixture comprising particulate zinc. Typically, the zinc mixture contains mercury and a gelling agent and becomes gelled when electrolyte is added to the mixture. The electrolyte is usually an aqueous solution of potassium hydroxide, however, other aqueous alkaline electrolytes can be used. The closed end of the cathode casing (when the casing is held in vertical position with the closed end on top) typically has a raised portion near its center. This raised portion forms the positive terminal and typically contains a plurality of air holes therethrough. The cathode casing closed end also typically has an annular recessed step which surrounds the positive terminal.
The cathode casing contains an air diffuser (air filter) which lines the inside surface of the raised portion (positive terminal contact area) at the casing's closed end. The air diffuser can be selected from a variety of air permeable materials including paper and porous polymeric material. The air diffuser is placed adjacent air holes in the raised portion of the casing closed end. Catalytic material typically comprising a mixture of particulate manganese dioxide, carbon and hydrophobic binder can be compacted into a disk shape forming a cathode disk within a cathode assembly. The cathode assembly with cathode disk therein can then be inserted into the cathode casing over the air diffuser on the side of the air diffuser not contacting the air holes. Typically a cathode assembly is formed by laminating a layer of electrolyte barrier material (hydrophobic air permeable film), preferably Teflon (tetrafluoroethylene), to one side of the catalytic cathode disk and an electrolyte permeable (ion permeable) separator material to the opposite side of the catalytic cathode disk. The cathode assembly with cathode disk therein is then typically inserted into the cathode casing so that its central portion covers the air filter and a portion of the electrolyte barrier layer rests against the inside surface of the step. The cathode disk in the final cell contacts the cathode casing walls.
If the cell is not adequately sealed, electrolyte can migrate to the edge of the catalytic cathode assembly and leakage of electrolyte from the cathode casing can occur. The leakage, if occurring, tends to occur along the peripheral edge of the cathode catalytic assembly and the cathode casing and then gradually seep from the cell through the air holes at the cathode casing closed end. The potential for leakage is greater when the anode casing and cathode casing is of very thin wall thickness, for example, between about 2 and 5 mil (0.0508 and 0.127 mm). Such low wall thickness is desirable, since it results in greater internal cell volume. However, there is a greater tendency for very thin walled cathode casing to relax or “spring back” after the cell is closed by crimping. Such relaxation can result in the development or enlargement of microscopic pathways between the cathode catalytic assembly and the inside surface of the cathode casing, in turn providing a pathway for electrolyte leakage.
The cathode casing can typically be of nickel plated cold rolled steel or nickel clad stainless steel with the nickel layer preferably on both inner and outer surfaces of the cold rolled or stainless steel. The anode casing can also be of nickel plated stainless steel, typically with the nickel plate forming the casing's outside surface. The anode casing can be of a triclad material composed of stainless steel having an outer layer of nickel and an inner layer of copper. In such embodiment the nickel layer typically forms the anode casing's outside surface and the copper layer forms the anode casing's inside surface. The copper inside layer is desirable in that it provides a highly conductive pathway between the zinc particles and the cell's negative terminal at the closed end of the anode casing. An electrical insulator seal ring of a durable, polymeric material can be inserted over the outside surface of the anode casing side wall. The insulator ring is typically of high density polyethylene, polypropylene or nylon which resists flow (cold flow) when squeezed and also resists chemical attack by alkaline electrolyte.
After the cell is assembled a removable tab is placed over the air holes on the surface of the cathode casing. Before use, the tab is removed to expose the air holes allowing air to ingress and activate the cell.
With more conventional anode and casing wall thicknesses which are greater than about 6 mil (0.152 mm), for example, between about 6 and 20 mil (0.152 mm and 0.508 mm), typically between about 6 and 10 mil (0.152 and 0.254 mm), the outside diameter of the anode casing with insulator seal thereon is typically less than the inside diameter of the cathode casing. (Such dimensions are as measured prior to pushing the cathode casing over the anode casing during assembly). In such case there will actually be some free space (zero interference) between the outside surface of the insulator seal and the inside surface of cathode casing wall when the anode casing and cathode casing are pushed together during the assembly step (prior to crimping). This is demonstrated, for instance, in U.S. Pat. No. 3,897,265 (Jaggard), Example at col. 6, lines 59–68. In the Jaggard Example the cathode casing had a wall thickness of 0.020 inches. The inside diameter of the cathode casing was 0.440 inches. The outside diameter of the anode casing was 0.410 inches. The insulator seal had a thickness of 0.010 inches. Thus, the outside diameter of the anode casing with seal thereon was 0.430 inches, which is less than the inside diameter of the cathode casing of 0.440 inches, after the anode and cathode casing were pushed together (pre crimp).
It has been conventional to assemble the anode and cathode together in two steps a) assembly and b) crimping. In the assembly step the cathode casing is pushed over the anode casing without any reduction occurring in the cell diameter. In the crimping step the assembled cell is passed through a crimping die wherein the overall cell diameter is typically reduced slightly and simultaneously therewith the edge of the cathode casing wall is crimped over the insulator seal and anode casing to provide a tightly sealed cell.
There has been effort more recently to reduce the wall thickness of the anode and cathode casing for zinc/air button cells. Such wall thicknesses can be reduced to values below the more conventional 6 mil (0.152 mm) to 20 mil (0.508 mm) level. For example, in U.S. Pat. No. 5,582,930 (Oltman) zinc air button cells having anode casing wall thicknesses between about 0.114 and 0.145 mm (4.49 and 5.71 mil) and cathode casing wall thicknesses between about 0.114 and 0.155 mm (4.49 and 6.10 mil) are recited (col. 4, lines 26–33). In U.S. Pat. No. 6,436,156 (Wandeloski) zinc/air button cell anode and cathode casing wall thicknesses between about 1 and 15 mil (0.0254 mm and 0.381 mm) are recited. Thus, anode and cathode casings as low as even about 1 mil (0.0254 mm) were being contemplated.
Zinc/air button cells having anode and cathode casing of reduced wall thickness are desirable, since more internal volume becomes available for active material, thereby extending the cell's service life. However, there are greater challenges in providing a tight, durable seal for such zinc/air button cells having small casing wall thicknesses, for example, between about 2 and 5 mil (0.0508 and 0.127 mm) than in cells with more conventional casing wall thicknesses, for example between about 6 and 20 mil (0.152 and 0.508 mm) and higher.
One difficulty in designing a tight, durable seal of such zinc/air button cells having anode and cathode casings of small wall thicknesses, for example, between about 2 and 5 mil (0.0508 and 0.127 mm), desirably between about 2 and 4 mil (0.0508 and 0.102 mm) is the tendency of the cathode casing side walls to relax or spring back after the cell has been assembled and the cathode casing edge has been crimped over the insulator seal wall and anode casing. Such spring back effect tends to cause a loosening in the seal interface between anode and cathode casing, thereby providing a pathway for the alkaline electrolyte to leak from the cell.
Another challenge in assembling the zinc/air cell, for example, in pushing the cathode casing over the anode casing in the assembly step (before crimping), is that the thin casing walls can become easily distorted if the assembly is not done very carefully. For example, in the conventional assembly step it has been the practice to apply a flat punch which pushes against the flat central portion of the closed end of the anode casing while another punch pushes in opposite direction on the cathode casing. This is illustrated, for example, in U.S. Pat. No. 5,658,356 (Burns) wherein flat punch 48 is shown to have a flat surface which impacts flush against the closed flat end 52 of anode can 54 while another punch 46 pushes against the closed end 22 of cathode can 20. Also in the crimping step (following assembly) it has been the practice to apply a flat punch flush against the flat closed end of the anode can while another punch pushes in opposite direction against the closed end of the cathode can. This is illustrated, for example, in U.S. Pat. No. 3,897,265 (Jaggard) which shows at FIGS. 9 and 9a that a punch 90 having a flat surface 92 impacts against the closed end of the anode casing of cell 10 with a force F2 while another punch 82 presses down on the closed end of the cathode casing with an opposing greater force F1. This results in cell size reduction and crimping of the cathode casing as the cell is pushed through die cavity 81. While such assembly and crimping technique may be suitable when applied to conventional anode and cathode casing, for example, having wall thicknesses between about 6 and 20 mil (0.152 and 0.508 mm) it is unsuitable for assembly and crimping of anode and cathode casing of very small thickness for example, between about 2 and 5 mil, (0.0508 and 0.127 mm).
Specifically, when the conventional technique is applied to such small walled anode and cathode casings the flat punch against the flat central portion of the closed end of the anode casing tends to cause an inward deformation (inward concavity) of the closed end of the anode casing during the assembly step and in particular during the crimping step. Since the closed end of the anode casing also functions as the cell's negative terminal, such inward concavity of the anode casing closed end is undesirable because it is desirable to have this end of the cell flat in order to assure a close and uniform contact between said negative terminal and the device being powered. Also, any inward concavity of the anode casing closed end may reduce the cell's available internal volume for active material.
Another challenge associated with good design for thin walled zinc/air cells is to assure that close and consistent contact is established and maintained between the cathode disk and the inside surface of the cathode casing. This must be accomplished in view of the tendency of the cathode casing to relax and spring back after cell assembly and crimping. The “spring back effect” is more pronounced as above mentioned when the anode and cathode casing wall thicknesses are reduced to low levels, for example, between about 2 and 5 mil (0.0508 and 0.127 mm). The cathode disk is inserted into the cathode casing before the cathode casing is pushed over the anode casing. The cathode disk is inserted so that it abuts the closed end of the cathode casing with the peripheral edge of the disk facing the inside surface of the cathode casing. If the cathode disk is sized so that it is of the same or less diameter as the inside diameter of the cathode casing, the spring back effect may reduce the uniformity of contact between the cathode disk edge and the cathode casing. The spring back effect may also produce a pathway for leakage of electrolyte around the cathode disk edge. On the other hand if the cathode disk is designed so that it has a diameter very much greater than the inside diameter of the cathode casing, then distortion and surface rippling of the cathode disk may occur. This also can cause leakage of electrolyte from the interior of the cell.
It is desired to produce a zinc/air cell which stays tightly sealed after the cathode casing wall has been crimped over the anode casing with insulator material therebetween.
It is desired to produce a zinc/air cell which maintains close contact between the edge of the cathode disk and the inside surface of the cathode casing wall.
It is desired to produce a zinc/air cell which remains tightly sealed, thus reducing the chance of electrolyte leakage, though the anode casing and cathode casing have very small wall thicknesses, for example, between about 2 and 5 mil (0.0508 and 0.127 mm).